There'll always be assholes looking to take best possible advantage of their situation at the expense of others.
Behavior is determined by environment. Change the environment and you change behavior.
If the con artists grew up in an environment where they were distrustful of others and prompts them to 'earn a living' and resort to 'shady practices' in order to survive, then yeah, you'd get that kind of behavior.
But in TNG, humans have "evolved" past all that. Halfway to Salamanders!
Sarcasm?
Just because humans have moved past what is currently perceived as the 'norm' doesn't make it impossible or improbable.
Its very much likely to occur in reality too if we got off our rear ends and used our science and technology to solve existing problems (which has been more than doable for decades).
In fact, in reality, we don't have scarcity issues. What we do have are distinct lack in management of Earth's resources, using an outdated socio-economic system and 'forms of government' (filled with idiots who are not problem solvers) and a systems value disorder (which is a product of living in the said system).
If you lived in an environment where you didn't have to worry about 'working for a living', 'paying bills' or earning money in general to survive, to go somewhere or DO anything, your outlook on things changes.
You wouldn't be concerned (or obsessed) about acquiring material goods anymore because they would be accessible to you for use whenever you needed them.
So at this point, you'd be concerned mostly with where would you like to travel to... study perhaps (fields of interest), how to possibly further improve the standard of living for everyone beyond what is already there and go actually DO stuff you find enjoyable and 'contribute' to society in that way (not through the notion of 'working a job').
In such a society, how one contributes is fundamentally redefined and has little or nothing to do with how we do things now.
I know its a 'tough' concept for some to swallow, but that's a problem of living in the society we do because its filling people's heads with stupidity, detracts from the problems we have in the world, creating mindless 'diversions' all to keep the money game going (for the most part).
And if you start delving into politics, you start realising just what a circus show it really is... filled with proverbial clowns who don't know jack about anything of actual relevance.
I mean TNG did have Shelby for example, who did use a few tricks in an attempt to get Riker's position whether he wanted to give it up or not.
What Star Trek seems to propose (at least once Season 1-2 of TNG had passed) is that the removal of things like poverty through technology has stopped people existing in a struggle for survival against each other.
So it leads to people being in competition for things they want (like being First Officer on the Flagship) rather than things they desperately need (food and shelter).
And cultural shifts like most of humanity in the 24th century striving for self-improvement rather than money are always gonna happen, and with the miraculous technology seen in Star Trek cultural shifts would be inevitable.
Here's the thing... you don't need the 'miraculous technology of Trek' to do that in real life. All you need really is sufficiently advanced means to generate abundance for everyone... and we had those methods for a good part of the last 100 years now.... we area also already generating abundance, but we have massive inefficiency in regards to management and distribution of said resources (capitalism),
Our methods of generating abundance are also heavily outdated (not in line with what could feasibly be done if we used state of the art methods) and as such require more land mass (even though we could easily reduce our footprint on Earth by 10 times at the very least).
